If you intended “John Sorilla, brother of the subject, is the co-maker of the principal borrower” as a complete sentence, then I don’t see why your officemate insisted on dropping the linking verb “is” from that construction. It’s a letter-perfect and grammatically perfect sentence as it is; without “is,” the sentence becomes simply a noun phrase: “John Sorilla, brother of the subject, the co-maker of the principal borrower.”
(http://josecarilloforum.com/imgs/linking-verbs_act-chairn-links-1A1.png)
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But then we must view this matter in its total context. Was your sentence part of a narrative, exposition, or dialogue, or was it a stand-alone entry at the bottom of, say, a document meant to identify “John Sorilla”? If this was the case, you’re absolutely right in insisting to retain the linking verb in your construction. On the other hand, if the construction was simply meant as a label like, say, “John Doe, brother of the accused, defendant,” then there was really no need for that linking verb. The phrase could stand by itself to do the identifying job without need for a verb.
On the basis of the above considerations, I think you can find common grammatical ground with your officemate and not allow this issue to needlessly rankle between the two of you.
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How English auxiliary verbs differ from linking verbs (http://josecarilloforum.com/forum/index.php?topic=8168.0)